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Fossils and genetics.

Today I would like to share a little information I have picked up while writing up some of my work for publication.

Collections of fossils, such as the collection of coral reef fossils that I mentioned last week are very useful for various areas of scientific study. The first area I am going to write about is phylogenetics: this is the study of evolutionary relationships between organisms using DNA evidence: “phylo” being a name for a “race’, “tribe” or “kind”, and “genetics” being the science of heredity dealing with resemblances and differences between related organisms resulting from the interaction of their genes (or DNA) with the environment in which they live.

When working out when one species evolved from another, or alternatively when two species evolve from one parent species, or when a particular character evolved in a group of organisms (e.g. wings of insects), the amount of change observed (between species or between character states) can be quantified using DNA evidence, and molecular clocks are used in this kind of analysis to show the amount of time required for a specific change in DNA to occur.

A molecular clock is basically a measure of how often nucleotide bases (A, G, C or T) in the organism’s DNA changes from one base to another, i.e. how long it takes for one feature of an organism to change into another. Certain molecules coded for by DNA appear to change between species at very similar rates. This was noted as the “genetic equidistance phenomenon” which infers equal time of genetic separation of two sister-groups (more closely-related groups) from an out-group (a less closely-related group) e.g. the divergence of taxa such as mammals, birds from fish. Mammals and birds will both have a similar genetic/molecular distance from fish, since fish diverged from the main vertebrate lineage earlier than either of the other two taxa, effectively giving a fish group and a mammal+bird ancestor group (the members of which would probably be a group of ancient amphibians, which later evolved into reptiles, which are the ancestors of both mammals and birds).

Any molecular clock must be calibrated using evidence from the fossil record of the species in question, since a molecular clock can only say that one time period of genetic change is say, twice as long as another, rather than giving a precise time in years (or in the above example, millions of years). Therefore to produce a molecular clock with actual dates for a particular difference or differences between living (or extant) organisms (measured in nucleotide changes in their DNA sequences), the time at which the organisms diverged from one another, or the time when a particular character was first evolved must be known, and this information can only be observed first-hand in the fossil record.

This is only a basic description of how molecular clocks are worked out and calibrated. There is much more literature on this subject, including reasons why molecular clocks can be inaccurate in particular cases. However molecular clocks can also be useful in providing approximate dates for events not evidenced in the fossil record (since the fossil record is a patchy representation of the history of life on earth, due to the conditions required for fossilisation to occur). However I hope this shows how fossils can be used in the field of phylogenetics.

The coral fossils I have found in SE Asia will hopefully be useful for calibration of molecular clocks in the expanding field of coral phylogenetics, which has gained much momentum over the last 10-15 years.